Flawless finish is for the very end of the grow - don't use it until the week before harvest.
You should switch from grow to bloom nutrients after the initial stretch has taken place. If it was a photoperiod I'd say wait 2 weeks after you flip 12/12. If you're growing auto's, wait at least a week after the first appearance of pistils. If you switch too early your plant will not have enough nitrogen for the stretch.